Hydroformylation of olefins in the presence of rhodium complexes and trisubstituted phosphines is well known. The process is used commercially to manufacture n-butyraldehyde from propylene, and 4-hydroxybutanal from allyl alcohol. While rhodium catalysts are favored for their high activity, the catalysts deactivate over time and require regeneration or replacement. Because rhodium is so expensive, catalyst reactivation is needed to make the process commercially viable.
One way to prolong the lifetime of rhodium hydroformylation catalysts is to include a diphosphinoalkane in the catalyst system as is taught by Matsumoto et al. in U.S. Pat. No. 4,215,077. However, even when a diphosphinoalkane is used, catalyst lifetimes are shorter than desirable, and regeneration is needed.
Numerous processes have been suggested for reactivating rhodium hydroformylation catalysts. One process uses a distillation to concentrate rhodium catalyst residues, treats the residues with oxygen, and washes the catalyst system with water or aqueous base (see U.S. Pat. Nos. 4,297,239 and 4,374,278). In another process, the deactivated catalyst system is separated from the aldehyde products by extraction and is exposed to syngas (a gaseous mixture of hydrogen and carbon monoxide) to reactivate the catalyst (U.S. Pat. No. 4,537,997). In another process, the aldehyde content of the catalyst system is adjusted, the catalyst is then treated with an oxygen-containing gas, solid oxides are filtered out, and ligands (e.g., triphenylphosphine) are added to regenerate the catalyst (U.S. Pat. No. 4,196,096).
Although each of these processes has merit, none is satisfactory for regenerating catalyst systems that include a rhodium hydridocarbonyl tris(trisubstituted phosphine) complex, a trisubstituted phosphine, and a diphosphinoalkane. Simply treating the deactivated system with syngas is ineffective. Air oxidation of the system is helpful, but by itself, is inadequate. Aqueous washing of catalyst generates unwanted waste streams that require costly treatment and disposal. A process that allows reactivation and regeneration of these rhodium hydroformylation catalyst systems is needed. Especially desirable is a way of reactivating rhodium catalyst systems useful for allyl alcohol hydroformylation.